Method for production of radiation diagnostic imaging agent

ABSTRACT

A method for producing a radioactive diagnostic imaging agent which contains a radioactive halogen-labeled amino acid compound as an effective ingredient and has a composition that can inhibit radiolysis of the effective ingredient, thereby further improving stability, is provided. Disclosed is a method for producing a radioactive diagnostic imaging agent, comprising a solution preparation step of preparing a solution containing a radioactive halogen-labeled amino acid compound, and a dilution step of diluting the solution containing the radioactive fluorine-labeled organic compound in order to adjust radioactive concentration thereof, which further comprises an acid addition step of adding an acid to the solution containing the radioactive fluorine-labeled organic compound, after the solution preparation step and before the dilution step, wherein the acid is added in the acid addition step in an amount sufficient to adjust the pH of the solution resulting from the dilution step to 2.0-5.9, for example, in an amount corresponding to 0.40-2.8 mmol per 1 L of the solution resulting from the dilution step.

TECHNICAL FIELD

The present invention relates to a method for producing a radioactive diagnostic imaging agent containing a radioactive halogen-labeled organic compound as an effective ingredient. More specifically, it relates to a method for producing a radioactive diagnostic imaging agent, which can inhibits radiolysis of the radioactive halogen-labeled organic compound.

BACKGROUND ART

The radioactive diagnostic imaging agent is a medicine directly administered to a human body and is a pharmaceutical composition containing a compound labeled with a specific radioisotope as an effective ingredient. The radioactive diagnostic imaging agent enables diagnosis by administering an agent to a subject and detecting a radiation emitted from the compound, followed by imaging based on information obtained from the radiation. The thus-conducted diagnosing method is referred to as nuclear medicine examination, and is effective in diagnosing a variety of diseases including heart disease and cancer. Also, nuclear medicine examination is characteristic in that it has not only high specificity and sensitivity to diseases, but also has an advantage of providing information on the functionality of lesions, compared to other examination techniques.

Compounds which are researched and developed for radioactive diagnostic imaging agents include 3-[¹⁸F]fluoro-1-aminocyclobutanecarboxylic acid (hereinafter referred to as [¹⁸F]FACBC). It is known that [¹⁸F]FACBC is taken up into a cell via an amino acid transporter. Thus, [¹⁸F]FACBC is expected to be developed as a tumor diagnostic agent since it is largely taken up into tumor cells which are highly proliferative and active in protein synthesis.

In radioactive diagnostic imaging agents, a problem often arises such that compounds decompose by self-radiation during delivery of the agents so as to cause decrease in radiochemical purity due to so-called radiolysis. For general pharmaceuticals, it is recommended in the guideline of ICH that if decomposed matters in an agent exceed 1.0%, the decomposed matters be subjected to structure determination when the maximum daily dosage of an effective component thereof is as small as not more than 1 mg (Non-Patent Document 1). However, for radiopharmaceuticals low in a blended amount per se in the agent, in most cases, the physical amount of impurities resulting from the radiolysis in the agent is as small as about 10⁻¹² mol, even if it exceeds 1.0%. Thus, it is very difficult to presume a structure of the decomposed matters, and to conduct verification as to whether or not the decomposed matters affect effectiveness such as tumor accumulation of the agent.

Also, particularly in radioactive diagnostic imaging agents, if impurities are generated due to decomposition, they often significantly affect the resulting image, even if the generated amount thereof is a small amount. Therefore, impurities in the radioactive diagnostic imaging agent should be maintained as low as possible, and it is preferable that radiolysis which may cause the production of impurities should also be inhibited as much as possible.

Various methods for inhibiting radiolysis have been examined focusing on application to [¹⁸F]fluoro-[¹⁸F]FDG).

International Publication No. WO03/090789 pamphlet discloses a method of reducing the radiolysis of [¹⁸F]FDG by adding a weak acid-based buffer to a [¹⁸F]FDG solution and an injection prepared by the method (Patent Document 1). Also, International Publication No. WO04/043497 pamphlet discloses adding ethanol to a [¹⁸F]FDG solution to obtain a composition for injection which may be (Patent Document 2).

Japanese Patent Laid-open (Kokai) No. H10-147542 discloses a technique utilizing an organic compound high in physiological acceptability such as monosaccharides, disaccharides, organic acids and salts or esters thereof as a radiation protecting agent (Patent Document 3). In this publication, the organic compound high in physiological acceptability and particularly effective as the radiation protecting agent is defined to have a reaction rate constant with OH radicals, H radicals or hydrated electrons in the range of 1×10⁸ to 5×10¹⁰ mol⁻¹s⁻¹.

International Publication No. WO06/134822 pamphlet discloses a radioactive diagnostic imaging agent reduced in radiolysis by blending various sugar or sugar alcohol.

Non-Patent Document 1: Pharmaceutical Affairs Bureau Notification No. 0624001 (page 12) Patent Document 1: International Publication No. WO03/090789 pamphlet Patent Document 2: International Publication No. WO04/043497 pamphlet Patent Document 3: Japanese Patent Laid-open (Kokai) No. H10-147542 Patent Document 4: International Publication No. WO06/134822 pamphlet

DISCLOSURE OF THE INVENTION Problems to be Solved by the Invention

As described above, International Publication No. WO03/090789 pamphlet and International Publication No. WO04/043497 pamphlet disclose conditions for preventing radiolysis of [¹⁸F]FDG in the solution. However, these documents only disclose techniques for reducing radiolysis of [¹⁸F]FDG only, but do not disclose any technique for reducing radiolysis of a series of radioactive fluorine-labeled amino acid compounds such as [¹⁸F]FACBC.

Japanese Patent Laid-open (Kokai) No. H10-147542 discloses a technique utilizing an organic compound high in physiological acceptability as a radiation protecting agent for radiopharmaceuticals. However, it is not apparent which compound is selected as the organic compound high in physiological acceptability or how much the compound is added in order to prevent radiolysis of the series of radioactive fluorine-labeled amino acid compounds such as [¹⁸F]FACBC.

International Publication No. WO06/134822 pamphlet discloses a technique inhibiting radiolysis by blending various sugar or sugar alcohol with a radioactive diagnostic imaging agent. However, this publication also fails to disclose a condition for effectively inhibiting radiolysis of the series of radioactive fluorine-labeled amino acid compounds such as [18F]FACBC.

The present inventors have found that radiolysis can be reduced by adding a sugar lactone to a radioactive diagnostic imaging agent containing [¹⁸F]FACBC as an effective ingredient, and thus have filed a patent application (Japanese Patent Application No. 2006-304338). This invention is very useful in that an effective amount of the sugar lactone has been found for inhibiting radiolysis of [¹⁸F]FACBC.

However, since the sugar lactone is a compound having a cyclic ester in its molecule, it is hydrolyzed when it is used as an injection such as a radioactive diagnostic imaging agent. This problem cannot be avoided for sugar lactone. Also, ascorbic acid which is one of the sugar lactone is a compound which has a double bond in its molecule so as to exhibit a reducing action and thus can easily be oxidized by dissolved oxygen or the like. Thus, the use thereof requires removing oxygen as much as possible, and is problematic in that handling operation may become complicated.

The present invention has been made in view of the above circumstances, and has aimed at providing a method for producing a radiopharmaceutical containing a radioactive halogen-labeled amino acid compound as an effective ingredient as an injection having a composition which can inhibit radiolysis of the effective ingredient so as to further improve stability.

Means for Solving the Problem

As a result of investigations, the present inventors have found that the radiolysis of [¹⁸F]FACBC is reduced dependently upon pH. Particularly, it has been found that when the pH value is not more than 5.9, stability thereof is maintained even if there exist no pharmaceutical additives or buffers that prevent radiolysis, as opposed to the conventional knowledge.

The present inventors have also found that the radiolysis can be additionally reduced by adding a pharmaceutical additive that inhibits the radiolysis.

Based on these information, it has been found that the radiolysis during the production step and even after the preparation of the agent can be reduced by adding a certain amount of acid and optionally a pharmaceutical additive to a [¹⁸F]FACBC solution which has not been subjected to concentration adjustment during the production step.

According to the present invention, a method for producing a radioactive diagnostic imaging agent, comprising

a solution preparation step of preparing a solution containing a radioactive fluorine-labeled organic compound represented by the following formula (I):

and, a dilution step of diluting the solution containing the radioactive fluorine-labeled organic compound in order to adjust radioactive concentration thereof, which further comprises an acid addition step of adding an acid to the solution containing the radioactive fluorine-labeled organic compound, after the solution preparation step and before the dilution step, wherein the acid is added in the acid addition step in an amount sufficient to adjust the pH of the solution resulting from the dilution step to 2.0-5.9. The addition amount of acid is preferably 0.40-2.8 mmol per 1 L of the solution resulting from the dilution step, and more preferably 0.52-2.8 mmol per 1 L of the solution resulting from the dilution step.

As an acid to be added in the acid addition step, can used an acid having physiological acceptability. Preferably, ascorbic acid, benzoic acid, hydrochloric acid, acetic acid, citric acid, gentisic acid, oxalic acid and the like can be used, more preferably, ascorbic acid, hydrochloric acid or gentisic acid can be used, and particularly preferably, hydrochloric acid can be used.

The diluting step in the present invention is conducted, for example, by adding water, a physiological saline solution or a Ringer's solution to the solution containing the radioactive fluorine-labeled organic compound.

Sugar or sugar alcohol may be further added as an additive agent in each of the acid addition step and the dilution step. In this instance, as sugar and sugar alcohol to be added, various compounds can be used, and preferable sugar alcohol includes erythritol xylitol, sorbitol or mannitol. Addition amount of sugar or sugar alcohol is preferably adjusted to an amount that meets a concentration of not less than 0.5 mmol/L in the solution resulting from the dilution step.

Meanwhile, the amount of sugar or sugar alcohol does not have any upper limit from the viewpoint of inhibition of radiolysis, but form the viewpoint of safety in use for injection, it should be maintained to be not more than a certain concentration. First, the amount of sugar or sugar alcohol needs to be within a range that is acceptable for additive agents for injection, that is, the total dose thereof needs to be not more than an amount that is acceptable for pharmaceutical additives. This range is determined in consideration of an acceptable daily dose of each additive agent. For example, the maximum dose each of sugar and sugar alcohol reported in a literature in case of intravenous administration is mannitol: 1.2 g, xylitol: 200 mg, sorbitol: 1.5 g, glucose: 8 g, fructose: 900 mg, martose: 10 g, and lactose: 1250 mg (“Japanese Pharmaceutical Excipients Directory 2000” edited by Japanese Pharmaceutical Excipients Council, YAKUJI NIPPO, LTD., 2000). Therefore, in order to blend these sugars or sugar alcohols as additives, the addition amount is set so that the final dosage contained in the injection does not exceed this maximum dose. On the other hand, for use in injection, physical properties of injections including osmotic pressure and viscosity must be taken into consideration. For example, when they are blended with an injection intravenously administered in an amount of about 2.5-5.0 mL at a time, it may be blended at a concentration of not more than 50 mmol/L, more preferably not more than 20 mmol/L.

EFFECT OF THE INVENTION

According to the present invention, an acid and optionally a sugar or sugar alcohol are added to a radioactive diagnostic imaging agent containing a radioactive halogen-labeled amino acid compound as an effective ingredient in the production process thereof, and thus radiolysis of the effective ingredient can be inhibited during production and storage, thereby further improving stability.

BEST MODE FOR CARRYING OUT THE INVENTION

Hereinafter, a preferable embodiment of the method for producing a radioactive diagnostic imaging agent according to the present invention will be described, taking the case of producing a [¹⁸F]FACBC injection.

A method for producing a radioactive diagnostic imaging agent according to the preferable embodiment is a method comprising a solution preparation step of preparing a solution containing a radioactive fluorine-labeled organic compound, an acid addition step of adding an acid to the solution containing a radioactive fluorine-labeled organic compound, and a dilution step of diluting a solution resulting from the acid addition step to adjust its radioactive concentration.

The solution preparation step of preparing the radioactive fluorine-labeled organic compound containing solution comprises; a step of imparting a radioactive fluorine to a precursor (Step 1); a step of conducting deprotection of a compound to which a radioactive fluorine has been imparted (Step 2); and a step of conducting purification of a solution containing [¹⁸F]FACBC after the deprotection (Step 3).

Radioactive fluorine used for labeling can be obtained by a known method, for example, a method in which H₂ ¹⁸O enriched water is used as a target and exposed to proton bombardment. In this instance, radioactive fluorine exists in the H₂ ¹⁸O enriched water used as a target. The H₂ ¹⁸O enriched water containing radioactive fluorine is allowed to pass through, for example, an anion-exchange column so that the radioactive fluorine is adsorbed and collected on the column, thereby being separated from the H₂ ¹⁸O enriched water. Thereafter, a potassium carbonate solution is allowed to pass through the column to elute the radioactive fluorine, and the eluate is supplemented with a phase transfer catalyst and is evaporated to dryness, thereby activating the radioactive fluorine.

In step 1, the mixture containing the dried radioactive fluorine is dissolved in acetonitrile, and cis-1-(N-tert-butoxycarbonyl)amino-3-[(trifluoromethyl)sulfonyloxy]cyclobutane carboxylic acid ethyl ester, as a precursor, is added to the acetonitrile solution to allow them to react under heating. As a result, radioactive fluorine is added to the precursor, whereby trans-1-(N-tert-butoxycarbonyl)amino-3-[¹⁸F]fluorocyclobutane carboxylic acid ethyl ester is synthesized.

In step 2, the solution of trans-1-(N-tert-butoxycarbonyl)amino-3-[¹⁸F]fluorocyclobutane carboxylic acid ethyl ester obtained in step 1 is subjected to deprotection and deesterification to yield a solution containing [¹⁸F]FACBC as a target product. This step can be conducted by various methods, for example, a method of giving an acidic condition to the reaction solution. In this instance, the acidic condition can be given by various methods, for example, a method of adding hydrochloric acid to a solution containing trans-1-(N-tert-butoxycarbonyl)amino-3-[¹⁸F]fluorocyclobutane carboxylic acid ethyl ester. The amount of acid to be added is not particularly restricted as long as the amount can provide an acidic condition sufficient for the deprotection.

In step 3, purification of the solution containing [¹⁸F]FACBC which is obtained in step 2 is performed. The purification process to be used includes various processes such as a liquid-liquid extraction process and a column separation process. For example, a process in which the reaction solution is injected into HPLC to obtain a fraction containing [¹⁸F]FACBC and a process in which various solid phase columns are used can be used. The [¹⁸F]FACBC solution can be obtained in this step, and thus the solution preparation step of preparing a radioactive fluorine-labeled compound containing solution is completed.

After the solution preparation step of preparing a radioactive fluorine-labeled compound containing solution has been completed, an acid addition step is conducted. This step is conducted by adding an acid in an amount sufficient to have a radiolysis inhibition effect to the [¹⁸F]FACBC solution which is obtained in the above solution preparation step. Since the acid is added for the purpose of adjusting the pH of the radioactive diagnostic imaging agent, various acids acceptable for pharmaceutical additives can be used as the acid. Examples of acids preferably used include ascorbic acid, benzoic acid, hydrochloric acid, acetic acid, citric acid, gentisic acid and oxalic acid, and more preferably ascorbic acid, hydrochloric acid or gentisic acid is used, and particularly preferably hydrochloric acid is used. The amount of an acid to be added is an amount sufficient to make the pH of the solution after the dilution step to 2.0-5.9, and more preferably to 2.0-4.9, and preferably an amount corresponding to 0.40-2.8 mmol per 1 L of the radioactive diagnostic imaging agent resulting from the dilution step described later. For example, when the amount of solution resulting from the dilution step is 75 mL, the acid corresponding to 0.030-0.21 mmol (that is, 0.30-2.1 mL when the acid to be added has a concentration of 0.1 mol/L) may be added.

Meanwhile, the amount of solution resulting from the dilution step can be readily determined by methods usually employed by the skilled in the art. For example, it can be readily determined by a method in which the radioactive concentration of the [¹⁸F]FACBC solution obtained in the above solution preparation step is divided by a radioactive concentration of the target agent, followed by multiplying the obtained quotient by the amount of the [¹⁸F]FACBC solution obtained in the above solution preparation step.

After the acid addition step has been completed, the dilution step is conducted to obtain a radioactive diagnostic imaging agent as a target compound in the method for production of the present invention. The dilution step can be conducted by adding water, a physiological saline solution or a Ringer's solution so as to make the amount of solution determined as above. After the completion of the dilution step, the solution can be aliquoted in an aimed amount in a vial for use as the radioactive diagnostic imaging agent.

Meanwhile, when a radioactive diagnostic imaging agent further comprising sugar or sugar alcohol is prepared, sugar or sugar alcohol may be added to the [¹⁸F]FACBC solution in the acid addition step and/or the dilution step. Sugar or sugar alcohol can be added in a various form such as a solution, powder or crystal, but is preferably added in a form of solution from the viewpoint of workability.

The radioactive diagnostic imaging agent obtained by the present invention should have a radioactivity enabling PET imaging when it is used. For example, with the aim of performing PET imaging on adults, it should have a radioactivity of 50-225 MBq when it is used.

The obtained radioactive diagnostic imaging agent can be used in the same manner as other generally-known radioactive diagnostic imaging agents. Specifically, it can be administered intravenously or locally to a subject. Distribution of the administered agent can be imaged by using a PET device in accordance with a conventional method.

EXAMPLE

Hereinafter, the present invention is described in more detail by way of Examples and Reference Examples. However, the present invention is not limited to these Examples.

Examples 1-7

[¹⁸F]fluoride ion-containing H₂ ¹⁸O (with a radioactivity of 138-158 GBq at the initiation of production, see Table 1) was allowed to pass through an anion-exchange column to adsorb and collect [¹⁸F]fluoride ion on the column. Then, the column was washed with water, and a mixed solution composed of [¹⁸F]fluoride ion, a potassium carbonate solution and a phase transfer catalyst was obtained in accordance with an ordinary method (for example, a method described in the references (Radioisotopes, 50, (2001), p. 205-227; Radioisotopes, 50, (2001), p. 228-556; “Production and quality control of radioactive agents for PET—Handbook of synthesis and clinical use—(2^(nd) edition)”, edited by PET Chemistry Workshop).

After the obtained mixed solution was heated in a reaction vessel to evaporate water to dryness, a solution of cis-1-(N-tert-butoxycarbonyl)amino-3-[(trifluoromethyl)sulfonyloxy]-cyclobutane carboxylic acid ethyl ester in acetonitrile was added thereto. The obtained solution was heated so as not to evaporate acetonitrile, thereby allowing nucleophilic substitution reaction to proceed to obtain a [¹⁸F]fluorine-labeled compound.

After the reaction vessel was cooled to about 40° C., water was added to the reaction solution for dilution, and the mixture was passed through a reversed phase column to collect the [¹⁸F]fluorine-labeled compound. This column was washed with water, and flashed with a flow of helium gas. Then, the column was charged with a sodium hydroxide solution, and an alkali solution was eluated from the column and collected in a vial. This operation was repeated twice. Then, the column was washed with water, and then the washing was combined with the above-collected alkali solutions.

Next, to the above-collected solution, hydrochloric acid was added, and the mixture was heated to effect deprotection reaction. The mixture was then passed through an ion retardation column, an alumina column and a reversed phase column in this order to perform purification and obtain a stock solution of [¹⁸F]FACBC. The radioactivity and the solution amount of the obtained stock solution of [¹⁸F]FACBC were shown in Table 1.

TABLE 1 Radioactivity used in each experiment, and radioactivity and amount of [¹⁸F] FACBC stock solution Initial [¹⁸F] FACBC stock solution radioactivity Radioactivity* Amount of solution [GBq] [GBq] [mL] Example 1 138 32.1 14.7 Example 2 155 38.3 14.8 Example 3 151 34.0 15.6 Example 4 152 30.3 15.2 Example 5 156 37.1 15.3 Example 6 158 40.6 15.6 Example 7 150 38.4 14.7 *value expected 2 hours after the initiation of production

To a 16.5 mL vial, a hydrochloric acid solution containing mannitol at a concentration shown in Table 2 was aliquoted in an amount shown in Table 2, and further, each stock solution of [¹⁸F]FACBC shown in above Table 1 was wholly poured and mixed therein.

TABLE 2 A mannitol-containing hydrochloric acid solution added in each experiment and its addition amount Added mannitol-containing hydrochloric acid solution Addition Concentration Concentration Concentration amount of of added of mannitol of hydrochloric solution hydrochloric [mmol/L] acid [mmol/L] [mL] acid [mmol/L] Example 1 396.6 41.9 0.86 0.57 Example 2 662.2 101.9 0.49 0.67 Example 3 272.3 53.9 1.2 0.98 Example 4 276.9 53.4 1.2 1.1 Example 5 554.6 333.1 0.60 2.8 Example 6 550.1 338.5 0.59 2.5 Example 7 426.7 39.0 0.78 0.40 *Concentration of added hydrochloric acid is indicated as a concentration which resulted after the completion of the dilution step

To the solution obtained in the above step, a physiological saline solution containing mannitol at a concentration shown in Table 3 was added in an amount shown in Table 3 to make a sample solution. Meanwhile, each sample solution was prepared to have a mannitol concentration of 10 mmol/L.

TABLE 3 Content of mannitol in a mannitol-containing physiological saline solution used in the dilution step, addition amount of the physiological saline solution, and amount of the solution after the dilution step Content of Addition Amount of the mannitol amount of solution after the [mmo/L] solution [mL] dilution step [mL] Example 1 6.08 47.5 63.0 Example 2 7.11 59.9 75.2 Example 3 6.64 50.1 65.7 Example 4 5.82 43.2 58.4 Example 5 6.95 56.8 72.7 Example 6 7.44 63.8 80.0 Example 7 7.03 60.0 75.5

For each sample solution, a pH was measured using a pH meter (type: HM-30R, manufactured by DKK-TOA CORPORATION).

The results are shown in Table 4. As shown in Table 4, the samples (in Examples 1-7) of 10 mmol/L mannitol-containing final agent to which hydrochloric acid was added in an amount corresponding to 0.40-2.8 mmol/L had a pH value of 2.8-5.9 which was in the pH range having a radiolysis inhibition effect.

The above results have indicated that a final agent having a pH with a radiolysis inhibition effect can be prepared by adding a hydrochloric acid in an amount corresponding to 0.40-2.8 mmol per 1 L of a 10 mmol/L mannitol-containing final agent.

TABLE 4 Concentration of hydrochloric acid [mmo/L] pH Example 1 0.57 4.78 Example 2 0.67 3.93 Example 3 0.98 3.53 Example 4 1.1 3.51 Example 5 2.8 2.75 Example 6 2.5 2.77 Example 7 0.40 5.86 *Concentration of added hydrochloric acid is indicated as a concentration which resulted after the completion of the dilution step

Example 8-12, Comparative Example 1

A stock solution of [¹⁸F]FACBC was prepared in the same manner as in Example 1, and was left at room temperature for not less than 72 hours so as to attenuate radioactivity.

This solution was respectively aliquoted in an amount of 1.0 mL (Examples 8-9), 1.5 mL (Examples 10-12) and 2.5 mL (Comparative Example 1), and a hydrochloric acid solution containing mannitol at a concentration shown in Table 5 was added thereto in an amount shown in Table 5, followed by mixing.

TABLE 5 A mannitol-containing hydrochloric acid solution added in each experiment and its addition amount Added mannitol-containing hydrochloric acid solution Addition Concentration Concentration Concentration amount of of added of mannitol of hydrochloric solution hydrochloric [mmol/L] acid [mmol/L] [mL] acid [mmol/L] Example 8 229.0 58.0 0.090 2.4 Example 9 269.2 54.2 0.077 1.9 Example 10 360.0 45.4 0.087 0.52 Example 11 381.0 43.4 0.082 0.47 Example 12 404.6 41.1 0.078 0.42 Comparative 434.3 38.3 0.120 0.37 Example 1 *Concentration of added hydrochloric acid is indicated as a concentration which resulted after the completion of the dilution step

To the solution obtained in the above step, a physiological saline solution containing mannitol at a concentration shown in Table 6 was added in an amount shown in Table 6 to make a sample solution. Meanwhile, each sample solution was prepared to have a mannitol concentration of 10 mmol/L.

TABLE 6 Content of mannitol in a mannitol-containing physiological saline solution used in the dilution step, addition amount of the physiological saline solution, and amount of the solution after the dilution step Content of Addition Amount of the mannitol amount of solution after the [mmo/L] solution [mL] dilution step [mL] Example 8 0.00 1.1 2.17 Example 9 0.00 1.1 2.15 Example 10 7.33 6.0 7.55 Example 11 7.33 6.0 7.54 Example 12 7.33 6.0 7.54 Comparative 7.30 10.0 12.56 Example 1

The results are shown in Table 7. As shown in Table 7, the samples (in Examples 8-12) of 10 mmol/L mannitol-containing final agent to which hydrochloric acid was added in an amount corresponding to 0.42-2.4 mmol/L had a pH value of 2.9-5.3 which was in the pH range having a radiolysis inhibition effect. On the other hand, the sample (in Comparative Example 1) of 10 mmol/L mannitol-containing final agent to which hydrochloric acid was added in an amount corresponding to 0.37 mmol/L had a pH value of 6.0 which was outside the pH range having a radiolysis inhibition effect.

TABLE 7 Concentration of hydrochloric acid [mmo/L] pH Example 8 2.4 2.90 Example 9 1.9 3.09 Example 10 0.52 4.30 Example 11 0.47 4.72 Example 12 0.42 5.31 Comparative 0.37 5.96 Example 1 *Concentration of added hydrochloric acid is indicated as a concentration which resulted after the completion of the dilution step

Reference Examples 1-16, Comparative Examples 2-6; Relationship Between pH and Decrease of Radiochemical Purity

[¹⁸F]fluoride ion-containing H₂ ¹⁸O was allowed to pass through an anion-exchange column to adsorb and collect [¹⁸F]fluoride ion on the column. Then, the column was washed with water, and a mixed solution composed of [¹⁸F]fluoride ion, a potassium carbonate solution and a phase transfer catalyst was obtained in accordance with an ordinary method (for example, a method described in the references (Radioisotopes, 50, (2001), p. 205-227; Radioisotopes, 50, (2001), p. 228-256; “Production and quality control of radioactive agents for PET—Handbook of synthesis and clinical use—(2^(nd) edition)”, edited by PET Chemistry Workshop).

After the obtained mixed solution was heated in a reaction vessel to evaporate water to dryness, a solution of cis-1-(N-tert-butoxycarbonyl)amino-3-[(trifluoromethyl)sulfonyloxy]-cyclobutane carboxylic acid ethyl ester in acetonitrile was added thereto. The obtained solution was heated under stirring so as not to evaporate acetonitrile, thereby allowing nucleophilic substitution reaction to proceed to obtain a [¹⁸F]fluorine-labeled compound.

After the reaction vessel was cooled to about 40° C., water for injection was added to the reaction solution for dilution, and the mixture was passed through a reversed phase column to collect the [¹⁸F]fluorine-labeled compound. This column was washed, and flashed with a flow of helium gas. Then, the column was charged with a 4 mol/L sodium hydroxide solution, and the outlet of the column was closed. After three minutes passed, the outlet was opened to eluate an alkali solution from the column and collect it in a vial. This operation was repeated twice. Then, the column was washed with water, and the washing was combined with the above-collected alkali solutions.

Next, to the above-collected solution, hydrochloric acid was added, and the mixture was heated to about 60° C. to effect deprotection reaction. The mixture was then passed through an ion retardation column, an alumina column and a reversed phase column in this order to perform purification and obtain a stock solution of anti-previously placed in a vessel which received the stock solution of anti-[¹⁸F]-FACBC, so that the pH of the stock solution of anti-[¹⁸F]-FACBC was adjusted to about 3.5.

Radioactivity of the obtained stock solution of anti-[¹⁸F]-FACBC was measured, and then the stock solution was diluted with a physiological saline solution so as to have a radioactive concentration of about 510 MBq/mL at the time when experiment was initiated (0 hour in Table 9). 2.23 mL of this solution was aliquoted in a vial of 5 mL in volume, and a predetermined amount of a predetermined solution indicated in Table 8 was added thereto, to obtain a sample solution. The radioactive concentration of the sample solutions at the end of preparation was 653-686 MBq/mL.

TABLE 8 The solution added to each sample solution and pH at the end of preparation pH at the end Added solution (addition amount) of preparation Reference 500 mmol/L HCl (40 μL) 2.00 Example 1 Reference 500 mmol/L HCl (40 μL), physiological 2.05 Example 2 saline solution (40 μL) Reference 100 mmol/L HCl (50 μL) 2.66 Example 3 Reference physiological saline solution (80 μL) 3.41 Example 4 Reference physiological saline solution (50 μL) 3.46 Example 5 Reference 11 mmol/L NaOH (70 μL), physiological 3.97 Example 6 saline solution (10 μL) Reference 10 mmol/L NaOH (70 μL) 4.04 Example 7 Reference 12 mmol/L NaOH (70 μL), physiological 4.55 Example 8 saline solution (10 μL) Reference 11 mmol/L NaOH (70 μL) 4.58 Example 9 Reference 12 mmol/L NaOH (70 μL) 4.88 Example 10 Reference 17 mmol/L NaOH (60 μL), physiological 5.03 Example 11 saline solution (20 μL) Reference 13 mmol/L NaOH (70 μL) 5.11 Example 12 Reference 15 mmol/L NaOH (70 μL), physiological 5.46 Example 13 saline solution (10 μL) Reference 14.3 mmol/L NaOH (60 μL) 5.54 Example 14 Reference 17 mmol/L NaOH (70 μL), physiological 5.90 Example 15 saline solution (10 μL) Reference 14 mmol/L NaOH (70 μL) 5.94 Example 16 Comparative 18.5 mmol/L NaOH (60 μL), 6.28 Example 2 physiological saline solution (20 μL) Comparative 14.1 mmol/L NaOH (70 μL) 6.31 Example 3 Comparative 16 mmol/L NaOH (80 μL), physiological 6.57 Example 4 saline solution (10 μL) Comparative 15 mmol/L NaOH (70 μL) 6.83 Example 5 Comparative 17 mmol/L NaOH (80 μL), physiological 7.70 Example 6 saline solution (0 μL)

The sample solution was stored in an electric thermostatic chamber adjusted to 25° C., and was then subjected to TLC analysis which was performed on the following conditions at the time of initiation of the experiment (0 hour) and 8.5 hours after the initiation of the experiment. Values of radiochemical purity were calculated in accordance with the following equation (1). Measurement of the radiochemical purity was repeated three times for each sample solution.

TLC Analysis Conditions:

Mobile phase: acetonitrile/water/100% acetic acid=4/1/1 TLC plate: Silica Gel 60F₂₅₄ (trade name, thickness: 0.25 mm, manufactured by Merck & Co., Inc.) Mobile length: 10 cm TLC scanner: Rita Star (manufactured by Raytest) Number of analysis: Three times

$\begin{matrix} {{{Radiochemical}\mspace{14mu} {purity}\mspace{14mu} (\%)} = {\frac{{Radioactivity}\mspace{14mu} {{of}\mspace{14mu}\left\lbrack {\,^{18}F} \right\rbrack}{FACBC}\mspace{14mu} {peak}}{{Total}\mspace{14mu} {radioactivity}\mspace{14mu} {on}\mspace{14mu} {TLC}\mspace{14mu} {plate}} \times 100}} & (1) \end{matrix}$

The results are shown in Table 9 and FIG. 1.

TABLE 9 Changes of radiochemical purity and decrease of radiochemical purity of anti-[¹⁸F]-FACBC solution at each pH Radiochemical Decrease* purity (%) (%) pH 0 hour 8.5 hours 8.5 hours Reference 2.00 99.41 99.44 0.03 Example 1 Reference 2.05 99.59 99.42 −0.17 Example 2 Reference 2.66 99.38 99.33 −0.05 Example 3 Reference 3.41 99.51 99.21 −0.30 Example 4 Reference 3.46 99.39 99.26 −0.13 Example 5 Reference 3.97 99.48 99.02 −0.46 Example 6 Reference 4.04 99.38 99.11 −0.27 Example 7 Reference 4.55 99.56 99.01 −0.55 Example 8 Reference 4.58 99.35 98.98 −0.37 Example 9 Reference 4.88 99.44 99.07 −0.37 Example 10 Reference 5.03 99.46 98.89 −0.57 Example 11 Reference 5.11 99.51 98.93 −0.58 Example 12 Reference 5.46 99.52 99.06 −0.46 Example 13 Reference 5.54 99.49 99.03 −0.46 Example 14 Reference 5.90 99.51 98.86 −0.65 Example 15 Reference 5.94 99.45 98.93 −0.52 Example 16 Comparative 6.28 99.53 98.81 −0.72 Example 2 Comparative 6.31 99.37 98.80 −0.57 Example 3 Comparative 6.57 99.43 98.67 −0.76 Example 4 Comparative 6.83 99.32 98.35 −0.97 Example 5 Comparative 7.70 99.37 97.31 −2.06 Example 6 *Decrease (%) = (radiochemical purity after 8.5 hours) − (radiochemical purity after 0 hour)

Referring to the relation between the pH and the decrease of radiochemical purity, relatively mild decrease of radiochemical purity was observed with the pH increase from 2.00 to 5.94. The slope based on linear approximation was calculated, and as a result, the slope was −0.145 in the pH range of 2.00-4.88, and was −0.010 in the pH range of 5.03-5.94.

On the other hand, when the pH value was not less than 6.28, sharp decrease of radiochemical purity occurred with the pH increase. The slope based on linear approximation was calculated, and as a result, it was −1.000. This value was about 6.7 times the value in the pH range of 2.00-4.88, and about 100 times the value in the pH range of 5.03-5.94. These have indicated that when the pH value is not less than 6.28, a drastic decrease of radiochemical purity occurs compared with the pH range of 2.00-5.94.

Reference Examples 17-28 Relation Between Mannitol Concentration and Radiochemical Purity at pH Values of 3.44 and 4.78

A stock solution of anti-[¹⁸]-FACBC was prepared from [¹⁸F]fluoride ion-containing H₂ ¹⁸O in the same manner as in Reference Example 1. Then, to the prepared stock solution of anti-[¹⁸F]-FACBC, a hydrochloric acid and a physiological saline solution were added so as to have a radioactive concentration of about 500 MBq/mL and a pH value of about 4.8 at the time when experiment was initiated (0 hour in Table 11). 2.23 mL of the obtained solution was aliquoted in a vial of 5 mL in volume, and a mannitol solution or a hydrochloric acid at the concentration shown in Table 10 was added in an amount shown in Table 10 to obtain a sample solution. The radioactive concentration of the sample solution at the end of preparation was 553-565 MBq/mL.

TABLE 10 Addition amount of mannitol solution in each sample solution Mannitol concentration at the end of Added solution preparation pH (addition amount) (μmol/mL) Reference 3.44 Mannitol 0.83 mg/mL 0.1 Example 17 solution (50 μL) 40 mmol/L HCl (20 μL) Reference 3.44 Mannitol 4.17 mg/mL 0.5 Example 18 solution (50 μL) 40 mmol/L HCl (20 μL) Reference 3.44 Mannitol 8.34 mg/mL 1.0 Example 19 solution (50 μL) 40 mmol/L HCl (20 μL) Reference 3.44 Mannitol 41.72 mg/mL 5.0 Example 20 solution (50 μL) 40 mmol/L HCl (20 μL) Reference 3.44 Mannitol 83.43 mg/mL 10.0 Example 21 solution (50 μL) 40 mmol/L HCl (20 μL) Reference 3.44 Mannitol 166.87 mg/mL 20.0 Example 22 solution (50 μL) 40 mmol/L HCl (20 μL) Reference 4.78 Mannitol 0.83 mg/mL 0.1 Example 23 solution (50 μL) Reference 4.78 Mannitol 4.17 mg/mL 0.5 Example 24 solution (50 μL) Reference 4.78 Mannitol 8.34 mg/mL 1.0 Example 25 solution (50 μL) Reference 4.78 Mannitol 41.72 mg/mL 5.0 Example 26 solution (50 μL) Reference 4.78 Mannitol 83.43 mg/mL 10.0 Example 27 solution (50 μL) Reference 4.78 Mannitol 166.87 mg/mL 20.0 Example 28 solution (50 μL)

The sample solution was stored in an electric thermostatic chamber adjusted to 25° C., and the value of radiochemical purity was calculated in the same manner as in Reference Example 1 at the time of initiation of the experiment (0 hour) and 8.5 hours after the initiation of the experiment. Measurement of the radiochemical purity was repeated three times for each sample solution.

The results are shown in Table 11 and FIG. 2. In all the Reference Examples at the pH values of 3.44 and 4.78, decrease of radiochemical purity was drastically inhibited with increase of mannitol concentration, and the reduction effect was saturated at a mannitol concentration of not less than 5.0 μmol/mL.

Also, the decrease of radiochemical purity was more inhibited at both mannitol concentrations at the pH value of 3.44 than 4.78.

From the above results, it was confirmed that the pH value of the solution contributes to radiochemical stability. Also, it was shown that radiolysis can be additionally reduced by adding mannitol.

TABLE 11 Changes of radiochemical purity and decrease of radiochemical purity of anti-[¹⁸F]-FACBC solution in the presence of mannitol Radiochemical Decrease* Mannitol purity (%) (%) Concentration 0 8.5 8.5 pH (μmol/mL) hour hours hours Reference 3.44 0.1 99.38 99.10 −0.28 Example 17 Reference 3.44 0.5 99.46 99.22 −0.24 Example 18 Reference 3.44 1.0 99.39 99.25 −0.14 Example 19 Reference 3.44 5.0 99.47 99.42 −0.05 Example 20 Reference 3.44 10.0 99.42 99.33 −0.09 Example 21 Reference 3.44 20.0 99.47 99.41 −0.06 Example 22 Reference 4.78 0.1 99.39 98.93 −0.46 Example 23 Reference 4.78 0.5 99.47 99.09 −0.38 Example 24 Reference 4.78 1.0 99.38 99.14 −0.24 Example 25 Reference 4.78 5.0 99.45 99.30 −0.15 Example 26 Reference 4.78 10.0 99.41 99.26 −0.15 Example 27 Reference 4.78 20.0 99.46 99.27 −0.19 Example 28 *Decrease (%) = (radiochemical purity after 8.5 hours) − (radiochemical purity after 0 hour)

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graph which shows a relation between the pH and the decrease of radiochemical purity.

FIG. 2 is a graph which shows a relation between the mannitol concentration and the decrease of radiochemical purity. 

1. A method for producing a radioactive diagnostic imaging agent, comprising a solution preparation step of preparing a solution containing a radioactive fluorine-labeled organic compound represented by the following formula (I):

and, a dilution step of diluting the solution containing the radioactive fluorine-labeled organic compound in order to adjust radioactive concentration thereof, which further comprises an acid addition step of adding an acid to the solution containing the radioactive 15 fluorine-labeled organic compound, after the solution preparation step and before the dilution step, wherein the acid is added in the acid addition step in an amount sufficient to adjust the pH of the solution resulting from the dilution step to 2.0-5.9.
 2. The method of claim 1, wherein the acid is added in the acid addition step in an amount corresponding to 0.40-2.8 mmol per 1 L of the solution resulting from the dilution step.
 3. The method of claim 1, wherein the dilution step comprises adding water, a physiological saline solution or a Ringer's solution to the solution containing the radioactive fluorine-labeled organic compound.
 4. The method of claim 1, wherein the acid added in the acid addition step is selected from the group consisting of ascorbic acid, benzoic acid, hydrochloric acid, acetic acid, citric acid, gentisic acid and oxalic acid.
 5. The method of claim 1, wherein the acid addition step comprises not only adding an acid, but also adding a sugar or sugar alcohol in an amount that meets a concentration of not less than 0.5 mmol/L in the solution resulting from the dilution step, and/or, the dilution step comprises not only adding a liquid for dilution, but also adding a sugar or sugar alcohol in an amount that meets a concentration of not less than 0.5 mmol/L in the solution resulting from the dilution step.
 6. The method of claim 5, wherein the sugar alcohol added in the acid addition step or dilution step is erythritol xylitol, sorbitol or mannitol. 